It may be of interest to share a conversation Dr. Wallace
and I had regarding this usage in 2 Thes. 2.

Dr. Wallace is in agreement that this APOSTASIA is not
related to the "departing from the faith" of 1 Tim. 4.
(A belief held by quite a few noted scholars.)

His contention is that Paul had verbally instructed them
regarding this APOSTASIA during his stay there, and that
the Thessalonians knew what he was referring to, but we
can only conjecture. In other words, this APOSTASIA would
be unknown to those outside of the church at Thess.

This is his current position.

Kenneth Wuest and E. S. English are very much in the minority
with their positions that APOSTASIA refers to the Rapture.

Whatever APOSTASIA meant to those at Thess, it certainly was
something overt, unique, and identifiable.